Abstract: The double-peak fusion rate of capsules for high yield implosion experiments on Shenguang Ⅲ prototype laser is studied by numerical simulation. The competition of energy gain and loss in the fusion region causes the double-peak phenomenon in fusion rate. During the time of fusion, the ions in fusion region obtain energy mainly due to compression, and lose energy mainly by the Coulomb collision with electrons. The shock wave moves back and forth in the DT fuel because of the reflections in the center and at the inner surface of glass, which causes the fluctuation in compression power. The competition between the compression work and Coulomb collision fluctuates the internal energy of ions, resulting in the double-peak fusion rate. The simulations of bangtime are in good agreement with experiments.